Inspection class Remote Operated Vehicles (ROVs) are typically used to position a video camera underwater. The ROV usually contains electronics that are connected to a base station by a wire tether. Motor driven propellers called thrusters are used to move the ROV.
Current ROVs, for example as described in U.S. Pat. No. 6,662,742, generally use separate thrusters to control motion in the horizontal and vertical planes. For example, a pair of thrusters mounted horizontally on the sides of the ROV can move the ROV forwards, backwards and control azimuth, while another thruster mounted vertically can move the ROV up and down.
Since motors are generally heavy, this configuration is not optimally efficient. When the ROV is moving in the horizontal plane, the vertical thruster is essentially dead weight, so that the power to weight ratio is diminished. The situation is typically worse when moving vertically because the multiple horizontal thrusters that are idle reduce the efficiency even further.
Another problem with ROVs relates to the electronics. Control circuitry, which is generally not waterproof, is often housed in a watertight box. This allows for access to perform reprogramming of the electronics, but causes a problem because opening and resealing the watertight enclosure may be time consuming.
A solution to this problem may be to run the reprogramming signals through the tether, but this has the disadvantage of adding to the size, weight and cost of the tether.
Also, it may be desirable to encapsulate the electronics in epoxy, eliminating the need for a watertight enclosure for the electronics. This solution has not typically been employed in past ROVs because once encapsulated, either the electronics cannot be reprogrammed, or as mentioned above the reprogramming wires must be run through the tether.
Another problem with existing ROVs is that in general an expensive tether is required. This is because the tether typically contains power wires, control wires and video cable. Since video is usually a coaxial cable and the power and control signals are not, the tether must contain both standard unshielded wires for power and control and shielded coaxial cable for the composite video.
A standard solution is to use a custom cable for the tether, but this adds to the cost of the ROV. Another solution heretofore employed is to put batteries in the ROV eliminating the need to run power through the tether. This allows a single coaxial cable to be used for the tether, carrying modulated video and control signals. The problem with this method is that the batteries add weight to the ROV and the modulation circuitry can be expensive.
A need therefore exists for a propulsion system for an ROV that improves the power to weight ratio while allowing motion in both the horizontal and vertical planes. The electronics should be reprogrammable without requiring a watertight box or additional reprogramming wires in the tether, and the tether should supply video to the base station without requiring coaxial wires.